Utility meters are devices that, among other things, measure the consumption of a utility provided commodity, such as electric energy, gas, or water, by a residence, factory, commercial establishment or other such facility. Utility service providers employ utility meters to track individual customers' usage of utility provided commodities. Utilities track customer usage for many purposes, including billing and tracking demand for the relevant consumed commodity.
Increasingly, utility service providers prefer utility meters that employ electronic circuitry to perform measurement and communications operations. Electronic circuitry reduces the number of moving parts required to perform measurement operations, resulting in increased accuracy as well as higher reliability. Further, a utility meter is typically installed at or near the facility or residence of each customer. As a result, service providers historically needed field technicians or “meter-readers” to obtain data from the remotely located utility meters. Such manual meter reading imposes significant labor costs and is vulnerable to transportation problems and human error. Electronic circuitry also addresses this problem by allowing utility meters to communicate metering data and other information (such as, for example, various diagnostic data) to remote, central facilities, whereby large numbers of utility meters may be read remotely without human meter-readers.
It has become increasingly desirable to employ accurate real-time clocks in utility meters having electronic circuitry. For example, some meters store time correlated data for the purposes of charging customers different rates depending on the times of day when particular amounts of electricity are consumed. Such metering operations are typically referred to as “time of use” metering. Time of use metering requires a highly accurate clock to ensure accurate billing.
Another type of metering that is sensitive to clock accuracy is demand metering. In demand metering, a customer may be charged based on the customer's highest usage rate over any demand period within a billing cycle. A demand period is a finite time period, such as 15 minutes or an hour. An inaccurate clock can substantially degrade demand meter data, thereby resulting in significant overcharging or undercharging.
While demand metering and time-of-use metering benefit to some degree from the use of an accurate real time clock as described above, the type of metering that may have the most need for accurate time-keeping is power quality metering. Power quality metering is a type of metering in which power quality data, for example, waveform data, may be recorded from time to time for analysis. Power quality data may include data generated at or around the time that a power quality event, i.e., a power surge or power sag, occurs. Power quality data may be used by utilities and consumers to, among other things, identify the cause and/or effect of a power surge or a power sag.
In particular, one example of a power quality meter is disclosed by U.S. Pat. No. 5,627,759 to Bearden et al. (hereinafter the “Bearden patent”), which is assigned to the assignee of the present invention and incorporated herein by reference. The Bearden patent describes a revenue meter that is also operable to, among other things, detect power quality events, such as a power surge or sag, and then report the detection of the power quality event to a utility or supplier.
One of the useful features of the meter disclosed in the Bearden patent is waveform capture. The meter disclosed in Bearden patent is operable to obtain waveform information regarding the voltage and/or current waveform at about the time a power quality event is detected. Such a feature is advantageous because the captured waveform may be analyzed to help determine potential causes of the event, the severity of the event, or other pertinent data.
One use of waveform capture feature is to analyze the waveforms from several meters on an electrical network after a power quality event in order to evaluate the propagation of the fault through the network. For example, if a power surge occurs over a portion of the power distribution system, then the utility may obtain captured waveforms from various meters on that portion of the network. The utility may then obtain information on how the power surge propagated through the network, as well as other information, by comparing the waveforms captured by the various meters.
One difficulty of performing analysis on the captured waveforms of several meters is temporally aligning the captured waveforms. In particular, to benefit from comparing the waveforms from several meters after a power quality event, it is important to temporally align or synchronize the captured waveforms. However, commonly used electronic clocks in electronic meters are not highly synchronized to each other, or in fact, to any external equipment.
In the past, clock circuits within revenue meters have been calibrated periodically using the line voltage signal, which oscillates at 60 Hz. While such a practice increases the accuracy of the meter clock circuits, the 60 Hz signal is not always dependable, and indeed may become unavailable during a power sag or power outage. Such a drawback is particularly problematic because power quality meters require accurate timing precisely for those times when the power line signal becomes unavailable.
U.S. Pat. No. 5,995,911 teaches the use of a GPS signal to synchronize the internal clock of a power sensor device. The drawback of relying on a GPS signal is that the GPS signal is not always available in real conditions.
What is needed, therefore, is an arrangement for keeping accurate time within a revenue meter that has increased reliability over the prior art designs described above.